The present invention relates to phase modulation in general and more particularly to an apparatus and structure for phase or frequency modulating telecommunication carrier signals with baseband signals.
As is well known in the art, baseband signals which contain the information to be transmitted and which will hereafter be referred to as message signals are modulated onto a carrier in order to facilitate or enable transmission over considerable distances.
Various modulation techniques are known including frequency modulation as well as other techniques which essentially employ the zero intermediate frequency (Zero IF) in implementing communication systems.
As is known, there are conventional ways to accomplish phase modulation at a desired carrier frequency. A first way is to modulate a local oscillator or a reference oscillator in a synthesizer loop by the information signal. A second way is to modulate an oscillator at non-zero intermediate frequency which is then translated to the desired carrier frequency by a third frequency. In any event, these methods suffer in the following respects.
In regard to the first method, the ability to maintain a linear relationship between the input signal amplitude and the carrier phase deviation when the carrier frequency is changed is extremely difficult, and secondly, additional complicated control circuitry is necessary to select the proper third frequency and additional radio frequency hardware is required which results in widespread interference problems. The Zero IF phase modulator does not require compensation correction when the RF frequency is changed since its voltage-to-phase function is independent of the RF frequency. The required extra RF circuitry would only be one mixer and a power combiner if the 90 degree of the RF phase shift is provided by the synthesizer (otherwise, a 90 degree power divider is also needed).
As indicated, the Zero IF approach has been widely employed in the prior art. U.S. Pat. No. 4,238,850 issued on Dec. 9, 1980 to R. A. Vance and entitled TRANSMITTER/RECEIVER FOR SINGLE CHANNEL DUPLEX COMMUNICATIONS SYSTEMS describes a transmitter receiver which can be used for a cordless telephone. The receiver produces direct conversion from the radio frequency. The local oscillator frequency is modulated by an audio signal to be sent, the modulated signals being passed through a dual splitting and combining network. As described in this patent, the receiver portion makes use of the Zero IF or direct conversion method of demodulation. Such a method is also described in British specification Ser. No. 1,530,602 published Nov. 1, 1978 to I. A. W. Vance.
U.S. Pat. No. 4,470,147 entitled RADIO RECEIVER WITH QUADRATURE DEMODULATION AND DIGITAL PROCESSING which issued on Sept. 4, 1984 to J. K. Goatcher and is assigned to the International Standard Electric Corp. depicts a radio receiver which divides the radio frequency input into two channels and in each channel mixes it with the carrier wave frequency. A quadrature shift in the mixed carrier wave frequencies enable baseband signals to be filtered from the mixer outputs. Essentially the analog quadrature signals are converted to digital form and processed digitally to reproduce the original modulating signal for AM, FM or PM transmissions. For single sideband modulation the mixing frequency is the sideband center frequency instead of the carrier frequency.
U.S. Pat. No. 4,476,585 entitled BASEBAND DEMODULATOR FOR FM SIGNALS issued on Oct. 9, 1984 to J. Reed and is assigned to the International Telephone and Telegraph Corp. This patent shows a modulator which is employed in a Zero IF system and uses a local oscillator for providing quadrature output signals at the center frequency of an FM signal to be demodulated. The demodulator has first and second mixers for separately mixing the FM signals with the quadrature signals to provide a first and second output signal, each in quadrature at the outputs of the mixers. These signals are low pass filtered. A demodulator is shown which includes third and fourth mixers with each mixer receiving at an input the output of one low pass filter. At another input, the mixers receive the third and fourth signals. The third and fourth signals are derived from mixing a variable controlled oscillator signal with the local oscillator quadrature signal. Essentially, the patent also shows a switching circuit which operates to alternate the third and fourth signals as applied to the input of the third and fourth means as well as the output as applied to the difference amplifier and uses an additional amplifier coupled to the outputs of the mixers so that one can utilize automatic gain control in a Zero IF system.
A particularly pertinent patent is U.S. Pat. No. 4,540,958 entitled ZERO IF FREQUENCY MODULATOR which issued on Sept. 10, 1985 to E. J. Neyens et al. and assigned to the International Telephone and Telegraph Corp. Essentially, this patent describes an apparatus for frequency modulating carriers with baseband signals, and it basically includes an arrangement for generating two quadrature related baseband signals in two separate channels wherein each of the quadrature related baseband signals is a sinusoidal function of the instantaneous angle of the message signal to be transmitted. The apparatus further includes an arrangement for producing a frequency modulated carrier from the quadrature related baseband signals. This arrangement includes a device that provides two quadrature related carrier frequency waveforms, two mixing devices, each for individually mixing one of the quadrature related carrier frequency waveforms with one of the quadrature related baseband signals in one of the channels, and a combining device which combines the output signals from the two channels into a single frequency modulated carrier frequency output signal.
As one can ascertain, there are many patents which relate to Zero IF systems such as U.S. Pat. Nos. 4,476,585; 4,480,327; 4,462,107; 4,488,064; 4,506,262; 4,521,892, 4,525,835, 4,322,851 and 4,254,503. Essentially, the main purpose of such a system is to provide a communication system wherein the amount of tuned circuitry employed is substantially reduced. In obtaining a reduction in the number of tuned circuits, one is therefore able to integrate large portions of the receiver and produce radio receivers which are extremely compact and reliable.
These receivers, as indicated, are employed in many areas such as in selective paging systems and so on. Thus to achieve such advantages, the design of such receivers is implemented according to the Zero IF technique. As indicated, in such a system there is present a receiver in which the local oscillator signals are in phase quadrature at the carrier frequency and are each separately mixed with the incoming audio modulated signals. The resulting signals have Zero IF with the two sidebands folded over on each other at the base band and extending in frequency from DC to the signal sideband width of the original signal.
As one can see from the above cited references, the concentration has mainly been in regard to implementing a receiver design or a transceiver design utilizing baseband circuitry. Baseband circuitry will operate at frequencies where integrated circuit technology is well established and therefore a transceiver which consists primarily of integrated circuits is relatively small and inexpensive. As indicated, the prior art has concentrated mainly in the receiver area of the Zero IF transceiver, and hence the above-noted techniques mainly involve demodulator design as a sine/cosine demodulator or analog methods employing phase locked loops. These approaches are apparent when reference is made to the above-noted patents. In general it is indicated that the transmitter has not received much attention. It is, of course, apparent that in order to obtain the benefits of integrated circuit technology it would be extremely desirable to implement a modulator employing digital techniques. This allows one to use digital IC technology to further reduce cost and improve operation.
Thus it is an object of the present invention to provide a modulator which can be employed in a Zero IF transmitter and which will operate in both phase and frequency modulation modes.
It is a further object to provide a modulator that can be reconfigured as a demodulator for phase modulated signals to thereby provide a simpler and inexpensive transceiver by further having the ability of utilizing common hardware in both the transmit and receive modes.